The present invention relates to microelectromechanical systems (MEMS) and in particular to MEMS devices employing beams supported for movement on flexible transverse arms.
MEMS devices are extremely small machines fabricated using integrated circuit techniques or the like. The small size of MEMS devices allows the production of high speed, low power and high reliability mechanisms. The fabrication techniques hold the promise of low cost mass production.
The parent applications to this present application describe a MEMS electrical isolator in which a beam is supported for longitudinal movement on a set of axially flexible arms, the latter of which are tied to a substrate. Motion of the beam caused by a MEMS actuator at one end of the beam, transmits a signal to a sensor positioned at the other end of the beam and separated from the actuator by an insulating segment.
The structure of a beam supported by transverse flexible elements provides an extremely simple and robust MEMS device. Nevertheless, the precision required for certain applications, particularly those related to sensors, may be difficult to achieve using mass-production integrated circuit processes.
The present inventors have recognized that the complex multicomponent integrated circuit materials from which MEMS devices are constructed, have widely varying coefficients of expansion which may create distortions and stress in the MEMS beam structure (particularly in the flexible arms supporting the beam) as the MEMS device cools from high processing temperatures, or when the MEMS devices is used at different operating temperatures, or when the MEMS device is subject to local self-heating from the conduction of current. These distortions and stresses limit the beam structure""s application to certain precision applications.
Accordingly, the present invention provides several techniques to compensate for such dimensional distortions and stress in beam-type MEMS devices, allowing mass-production of increasingly precise and accurate mechanisms. The present invention further provides methods of controlling the typical distortions in the flexible arms to provide increased functionality in beam-type MEMS devices.
In this regard, the invention provides improved methods of attaching the flexible arms that support the beam to the substrate. These attachment methods are augmented by enforcement of conditions of symmetry on the beam and its structure. Control of bowing of the transverse arms, discovered by the inventors in connection with their study of temperature induced distortions of the MEMS structure, is used to add bias or bi-stability or mechanical amplification to the MEMS device.
Specifically then, the present invention provides a MEMS system having a beam supported on flexible transverse arms to move longitudinally along a substrate wherein ends of the transverse arms removed from the beam are connected to the substrate by elements allowing transverse movement of the ends of the arms. This transverse movement may be provided, for example, by a flexible longitudinally extending wrist.
It is one object of the invention, therefore, to provide an attachment system for the transverse arms that accommodates transverse dimensional changes in the arms caused by temperature changes and which, if uncorrected, can cause buckling of the arms, stress stiffening of the arms, or offset of the beam from its null position.
The wrist elements may attach to the transverse arms via arcuate sections.
Thus, it is another object of the invention to eliminate points of concentrated stress at the arm ends.
The wrist elements may include serpentine sections, and/or the serpentine sections may be placed at the ends of the transverse arms where they are attached to the wrist elements.
Thus, it is another object of the invention to provide an attachment mechanism for the transverse arms that is both transversely and rotationally unrestrained so as to mimic a xe2x80x9cfree beamxe2x80x9d whose ends are unrestrained. Transverse arms that approximate a free beam provides a less stiff bending force with movement of the supported beam and avoid stress stiffening such as may change the dynamic characteristics of the MEMS device.
The beam may be supported at longitudinally opposed ends by pairs of transverse arms extending from either side of the beam and the wrist elements for the transverse arms may either all extend toward the center of the beam or all extend away from the center of the beam.
Thus, it is another object of the invention to balance any forces on the beam caused by a slight bowing of the transverse arms such as may be incurred by an expansion of those arms or other distortions by encouraging countervailing bowing. It is a further object of the invention to compensate for any Lorentz forces that may occur on the wrists when current is passed through the transverse arms. By facing the wrists in the same direction, a transverse balancing of Lorentz forces from the wrists is obtained.
The beam may be supported at its center by a pair of transverse arms extending from the beam on opposite sides of the beam and the wrist elements for the center transverse arm may extend in opposite longitudinal directions.
Thus it is another object of the invention to promote an S-shape bending for a transverse arm centered on the beam such as prevents any longitudinal biasing of the beam as would occur with an uninflected bowing. Such a central beam may have no current flowing through it to eliminate any issues with Lorentz forces.
The beam may be designed to stabilize at a dimension that places the respective pairs of transverse arms on either end of the beam in equal and opposite flexure: either bowing in or bowing out.
Thus, it is another object of the invention to balance any of the forces that may be placed on the beam by distortions in the lengths of the flexible arms.
The transverse arms may also be made of equal length. The points of attachment of the transverse arms to other than at ends of the beam may be centered between the points of attachment of the transverse arms at the end of the beam. The actuator and biasing structures for the beam may be placed at the end of the beam.
Thus, it is another object of the invention to enforce a longitudinal and transverse symmetry on the MEMS device so that other effects of dimensional distortion in the transverse arms and beam are balanced out.
In one embodiment, the beam may be supported on at least one pair of flexible transverse arms, which are bowed to present a force that increasingly resists longitudinal motion of the beam in a first direction up to a snap point after which the force abruptly decreases. The force may change direction after the snap point or keep the same direction.
Thus, it is another object of the invention to provide a bistable or monostable mode of operation of the beam device.
After the snap point, the bow may increasingly resist longitudinal motion of the beam in a second direction opposite the first direction up to a second snap point at which the force abruptly decreases. The second snap point may be different from the first snap point.
Thus, it is another object of the invention to provide for a hysteresis actuation of the beam using mechanical elements.
In a different embodiment, the beam may be supported by at least one flexible transverse arm, which is angled to also extend longitudinally. A sensor detecting transverse motion may receive the first transverse arm at an end removed from the beam.
Thus, it is another object of the invention to provide for a mechanical amplification of either the force or motion of the beam as transmitted to the sensor structure.
The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.